What you actually wanted to replay is just the first-parent history,
redoing the merges with their respective original second parent.
Jonathan (Cc'ed) had the same problem about a month ago.

The "first parent" thing isn't that well defined, criss-cross merges
combined with some unfortunate fast-forwards will easily make the
first-parent history become the one that you didn't mean (ask any
git-svn users that dared to use "git merge" without fully understanding
how git-svn uses the history ;-)), but it works in this special case.
And with that special case in mind (and admittedly probably not thinking
much further) I came up with this patch a few years ago:

It was not my itch to scratch though, so I never got around to get it
included, and of course it doesn't apply cleanly anymore. I tried to
give some hints to Jonathan ("hachi") on #git on how to update the patch
to make it apply again:

Re: git rebase --preserve-merges seems to not work

>> $ git lg --all
>> * 2c93a8a - (HEAD) c (11 seconds ago)
>> * 2b98bc6 - b (11 seconds ago)
>> * 6abf527 - (d) d (3 minutes ago)
>> | * 472fd93 - (bc-merge) Merge commit 'c' into HEAD (74 seconds ago)
>> | |\
>> |/ /
>> | * d132c87 - (c) c (3 minutes ago)
>> | * 4a88fd1 - b (3 minutes ago)
>> |/
>> * b576660 - (a, master) a (4 minutes ago)
> What you actually wanted to replay is just the first-parent history,
> redoing the merges with their respective original second parent.
> Jonathan (Cc'ed) had the same problem about a month ago.
>
> The "first parent" thing isn't that well defined, criss-cross merges
> combined with some unfortunate fast-forwards will easily make the
> first-parent history become the one that you didn't mean (ask any
> git-svn users that dared to use "git merge" without fully understanding
> how git-svn uses the history ;-)), but it works in this special case.
> And with that special case in mind (and admittedly probably not thinking
> much further) I came up with this patch a few years ago:
>
> http://marc.info/?l=git&m=119379735525213&w=2

I hand merged your change in to my Git installation. There is one part
of git-rebase--interactive that looks entirely different, so I would
need to look back through the history to determine how to properly apply it.

The good news is the patch does exactly what I want in this simple
case. I will have to try it on some more complicated hierarchies to see
what the end result is.

It's curious... whenever I ask around about issue X or feature Y, a
previously submitted patch can generally be found in the list archives. :)